One of the most important applications for crystal growing is in the area of semiconductor substrate material. This substrate material is available in ribbon form in which ribbons of large-grain polycrystalline or single crystalline material are grown. The ribbons, usually of silicon, have wide application in the semiconductor industry and are especially well adapted for use as solar cell substrates.
Several processes are described in the literature for the growth of crystalline ribbon from melt. In one process capillary action is used to feed molten material up through a die which is used to shape the growing ribbon. In order to exercise control over this process it is necessary to control the heat removal from the ribbon surface, the pulling speed, the average die temperature, and the precise temperature distribution along the entire die, most specifically at the edges. This process suffers principally from a lack of growth stability since the position of the edges of the ribbon are difficult to control and the ribbon often "freezes" to the die when the die temperature or some other growth variable momentarily fluctuates.
An additional problem encountered with this technique is that all the material flowing up through the die is incorporated into the solidified ribbon. Since this material includes impurities, unacceptable impurity levels can exist in the solidified ribbon. In contrast, when pulling a ribbon directly from a melt impurities tend to become segregated at the interface between the melt and the solidified ribbon, such that the impurities are rejected back into the melt and do not become incorporated into the solidified ribbon. Since the use of a die partially prevents such segregation of impurities, pulling a ribbon directly from a melt is a prerferred process.
U.S. Pat. No. 3,129,061 described another process for ribbon growth which is referred to as the web dendritic growth process. In this process dendrites, which are of the same materials as the ribbon, are grown into a melt supercooled at the ribbon edges in order to stabilize the ribbon edge position. The principal problem associated with this technique is the high degree of temperature control needed to maintain dendritic growth at the ribbon edges while maintaining conventional growth along the ribbon "web". (See also "Thermal Analysis of Solidification in Web-Dendritic Ribbon Growth", by Harrill, Rhodes, Faust, and Hilborn, Journal of Crystal Growth, Vol. 44; pps. 34-44, 1978). Other web dendritic techniques are illustrated in U.S. Pat. Nos. 3,298,795; 3,031,403; and 3,370,927. The last of these patents is directed to the angular pulling of continuous dendritic crystals.
By way of further background, a technique for growing a matrix structure of silicon is illustrated in an article by Theodore F. Ciszek and Guenter H. Schwuttke, entitled "Inexpensive Silicon Sheets for Solar Cells", NASA Tech Briefs, Winter 1977, pps. 432-433. In this technique a graphite screen is dipped into a liquid silicon bath and is then pulled from the bath. This produces a patterned sheet or film of liquid silicon which solidifies in the screen to produce a textured semicrystalline composite. It will be appreciated that the technique described by Ciszek, et al is not a crystal growing technique because silicon is first captured by capillary action in the graphite screen and is then held until such time as it solidifies. This differs from the growth of ribbon from a melt in which crystallization takes place at the face of the melt as the ribbon is withdrawn. In ribbon growth, grain boundaries generally exist perpendicular to the plane of the ribbon, while the Ciszek, et al technique generally produces randomly oriented grain boundaries which act as random carrier traps for impurities, resulting in devices which are less uniform in performance. The Ciszek et al technique also results in undesirably small grain sizes and in a non-uniform central web which is encumbered by the grid structure. Because of the use of the grid, the product of Ciszek, et al is not a flat sheet, and if the grid embedded in the semiconductor ribbon is not very precisely placed, shorting of p-n junctions created by diffusion can occur.
With respect to prior art crystal growing furnaces, reference is made to U.S. Pat. Nos. 3,639,718 and 3,865,554. Both of these patents describe furnaces suitable for batch-type crystal growth using the Czochralski method. These furnaces are adapted for processes requiring critical monitoring during crystal pulling and are not designed or adapted for continuous ribbon growth. More specifically, these furnaces are designed to carry out crystal growing with temperatures maintained to .+-.0.1.degree. C., with visual inspection and automatic control being a prerequisite for obtaining uniform crystal properties.